1. Pure swards of perennial ryegrass (Lolium perenne L. cv. Melle) or white clover (Trifolium repens L.cv. Blanca) were harvested daily at three and two stages of growth respectively, and offered to housed cattle. The grass diets comprised primary growth (May) and two later regrowths of contrasting morphology (i.e. leaf: stem values of 1.54 and 2.84 respectively), and were characterized by high contents of water-soluble carbohydrate and neutral- detergent fibre and comparable in vitro dry matter (DM) digestibilities (mean 0.80). Total nitrogen content was higher on primary growth grass (34 g/kg DM) than on regrowths (23 g/kg DM) but lower than values obtained for the two clover diets (38 and 43 g/kg DM, respectively). The clover diets had lower water-soluble carbohydrate contents than the grasses, comparable cellulose, but lower neutral-detergent fibre contents and in vitro DM digestibilities of 0.70 and 0.77 respectively.
2. The experiment lasted from May until August, during which time a total of twenty-one young Friesian steers (initial average live weight 130 kg) were used to determine both nutrient supply to the small intestine (twelve animals) and apparent digestibility (nine animals). Each diet was offered at three levels of DM intake (i.e. 18, 22 and 26 g/kg live weight). A further six steers, all fed at the rate of 22 g DM/kg live weight, were used to determine the metabolizable energy contents of the five diets by means of open-circuit calorimetry.
3. The three grass diets and the later-cut clover had, as intended, quite similar in vivo organic matter digestibilities, but that of the earlier-cut clover was lower, and this was associated with a large number of flower heads in this crop at the time of feeding.
4. On the clover diets, proportionately less of the ingested organic matter appeared to be digested in the rumen (0.40) compared with the grass diets (0.58) (P < 0.001). On the high-N primary grass and the clover diets, substantial rumen losses of N were detected (P < 0.01) compared with regrowth grasses.
5. The metabolizable energy content of the primary growth of grass was 12.2 MJ/kg DM, whilst the values for the other two grass diets were lower (11.6 MJ/kg DM), despite no marked decline in overall energy digestibility. Values for the two clover diets (mean 10.5 MJ/kg DM) were considerably lower than all values noted for the grasses.
6. The amount of absorbed non-ammonia-N expressed per MJ metabolizable energy averaged 1.24 g/MJ on the grass diets, with no apparent seasonal effects, compared with 1.79 g/MJ on the two clover diets.
7. Reasons for variations in nutrient supply and the consequence of considerable rumen losses of ingested N on the high-N diets are discussed, and the contribution that white clover can make to overall protein supply in grazing ruminants is established.